Intrinsic room-temperature ferromagnetism in a two-dimensional semiconducting metal-organic framework

The development of two-dimensional (2D) magnetic semiconductors with room-temperature ferromagnetism is a significant challenge in materials science and is important for the development of next-generation spintronic devices. Herein, we demonstrate that a 2D semiconducting antiferromagnetic Cu-MOF can be endowed with intrinsic room-temperature ferromagnetic coupling using a ligand cleavage strategy to regulate the inner magnetic interaction within the Cu dimers. Using the element-selective X-ray magnetic circular dichroism (XMCD) technique, we provide unambiguous evidence for intrinsic ferromagnetism. Exhaustive structural characterizations confirm that the change of magnetic coupling is caused by the increased distance between Cu atoms within a Cu dimer. Theoretical calculations reveal that the ferromagnetic coupling is enhanced with the increased Cu-Cu distance, which depresses the hybridization between 3d orbitals of nearest Cu atoms. Our work provides an effective avenue to design and fabricate MOF-based semiconducting room-temperature ferromagnetic materials and promotes their practical applications in next-generation spintronic devices.

measurements were conducted in 1 M KOH (aq) electrolytes continuously purged with 99.999% N2 (Praxair) and at a sweep rate of 5 mV/s.Then, Mott-Schottky analysis was carried out in the linear region of the C −2 curve from 0.9 to 1.3 V vs Ag/AgCl with a frequency of 2 kHz.The solid-state cyclic voltammetry was measured at 0.1 V/s scan rates in 0.1 M TBAPF6/DMF and KCl solutions.The black cross, red and green horizontal lines, and blue vertical lines are defined as experimental, calculated, difference data and marked position, respectively.The enlargement was marked as yellow shading.
2] Therefore, we synthesized the Cu-ABDC using 2amino-1,4-benzenedicarboxyulic acid (ABDC) and Cu(NO3)2‧3H2O, and the crystal structure is exactly consistent with that of well-known monoclinic Cu(tpa)‧DMF. 3We also have carried out the Rietveld refined XRD pattern of the Cu-ABDC MOF using the software Rietica.As seen from Figure S3, all peaks of the MOF are indexed to the standard structure Cu(tpa)‧DMF MOF with space group C 2/m (CCDC-687690), which is synthesized using 1,4-benzenedicarboxyulic acid (BDC) and Cu(NO3)2•3H2O and has been reported by Carson et al., [1][2][3][4][5] suggesting that the Rietveld refinement of the XRD data reveals an almost single-phase nature of the MOF.The lattice parameters of the MOF obtained from the refinement are a = 11.1345Å, b = 14.2439Å and c = 7.8529 Å, which is basically in agreement with the value reported by Carson et al., indicating a reliable quality for our sample.Additionally, in order to exclude the Cu-related phase in our samples, synchrotron radiation powder X-ray diffraction patterns of 1.4%-LS-Cu-ABDC and the reference sample CuO were also performed.Compared to the characteristic peak at about 35.5 o and 38.7 o for CuO, there are no obvious peaks at the same position for 1.4%-Cu-ABDC sample (Figure S3b), confirming the absence of CuO and hence intrinsic ferromagnetic in our samples.

a b
Supplementary Figure 5. FT-IR analysis.The curves of all the samples are normalized by the vibration intensity of C=C (marked as yellow shadings) for the same amount of the organic ligand.For all the Cu-MOFs, the peak at about 3482 cm −1 is clearly observed which is ascribed to the asymmetrical stretching vibration adsorption of the amine groups.In the lower frequency region, the peaks at about 1590, 1507 and 1259 cm −1 correspond to the N-H, C-H and C-N stretching bonds, respectively.The peaks at about 1101, 1500 and 1668 cm −1 are ascribed to the vibrations of C-O, C=C stretching of the benzene ring and C=O of solvent molecule. 8In addition, two peaks at ~1384 and 1619 cm −1 are assigned to the symmetric and asymmetric vibrations of the -COO groups, confirming the existence of ligand in Cu-MOFs skeleton. 9-10Besides, a characteristic peak at about 580 cm −1 can be assigned to Cu-O stretching, suggesting that organic ligands are efficiently coordinated to Cu atoms to form Cu-MOFs.
Moreover, the vibration intensities of -COO gradually decrease with the increase of ligand cleavages after the normalization with the signals of C=C, suggesting the missing of organic linkers.Obviously, the vibration intensities of -NH2 for 1.4%-and 2.7%-LS-Cu-ABDC gradually decrease with the increase of ligand cleavages by about 25% and 42%, respectively, which is close to the actual ratio of benzoic acid.Therefore, the chemical formula for Supplementary Figure 6.Raman spectroscopy analysis.The vibration intensity of all the peaks are normalized by the vibration intensity of C=C (marked as yellow shadings) at about 1633 cm −1 for the same amount of the organic ligand.In the lowfrequency region, a doublet at about 194 and 229 cm −1 are assigned to Cu-Cu stretching modes.At about 460 cm −1 , Cu-O stretching mode is detected, which indicates that Cu ions coordinate with ligands and the successfully synthesis of the Cu-MOFs. 11The peaks at ~820 and 1130 cm −1 represent the vibrations of C-H in Cu-MOFs.In addition, a peak at about 1267 cm −1 is attributed to C-O bond.3] Meanwhile, a new band can be discovered at about 1137 cm −1 , likely owing to a deformation mode involving the carboxylate group coupled with a C-C stretching mode. 14It is worth noting that the vibration intensities of -COO gradually decrease with the ligand cleavages, which further confirms the missing organic linkers.300 600 900 1200 1500 Raman Intensity (arb.units) Supplementary Figure 9. Cu K-edge EXAFS fitting curves in R-space.Cu K-edge EXAFS fitting curves in R-space of Cu-ABDC (a), 1.4%-LS-Cu-ABDC (b), 2.7%-LS-Cu-ABDC (c), respectively.The fitted R range here is from 1.1 to 2.4 Angstroms.
Cu-ABDC fitting Supplementary Figure 10.Cu K-edge EXAFS fitting curves in q-space.Cu K-edge EXAFS fitting curves in q-space of Cu-ABDC (a), 1.4%-LS-Cu-ABDC (b) and 2.7%-LS-Cu-ABDC (c), respectively.For Cu L-edge XAS in Fig. 2c, firstly the linear pre-edge background is subtracted, and the L3 and L2 edges were normalized on intensities of the corresponding L2 peaks in order to reveal the variation of the d-orbital occupation state more clearly.Two absorption peaks D (21453 cm −1 ) and E (13609 cm −1 ) can be detected, which are attributed to charge transfer between ligand and metal and metal-radical spinexchange originated from d-d transition. 18] Moreover, bandgaps are fitted as 1.20, 1.22 and 1.27 eV for Cu-ABDC, 1.4%-LS-Cu-ABDC and 2.7%-LS-Cu-ABDC, respectively, suggesting its semiconductor behaviors and the bandgaps gradually increase with the ligand cleavages, in agreement with theoretical calculation results below.Remarkably, the UV-Vis-NIR spectra for all the Cu-ABDC samples exhibit absorption at about 13609 cm −1 extending to the near-infrared (NIR) region. 8000 16000 24000 32000 40000 48000 Intensity (arb.units)Wavenumber (cm -1 ) Supplementary Figure 18.Mott-Schottky plots of Cu-MOFs.4] Typical negative slopes of the linear region in the Mott-Schottky plots can be founded, indicating all the Cu-MOFs exhibit p-type semiconductor character, 25 in agreement with theoretical calculation results.Additionally, the carrier concentration is inversely proportional to the slope of the plots and can be calculated from the function as stated in previous report. 23herefore, the hole carrier density of Cu-ABDC is about 2.5 and 3.5 times larger than that of the 1.4%-LS-Cu-ABDC and 2.7%-LS-Cu-ABDC, respectively.In order to further investigate the long-range ferromagnetic order, we preformed the in-phase and out-of-phase AC magnetic susceptibility χ′(T) and χ′′ (T) for 1.4%and 2.7%-LS-Cu-ABDC MOF as a function of temperature at an AC field of Hac = 2 Oe and Hdc = 0 Oe with several frequencies (f = 1, 10, 100, 500, and 1000 Hz) at the temperature range of 50-360 K.1][32] Meanwhile, AC magnetic susceptibility signals related to long-range order were also missing in the measurements, probably due to that the test temperature is well below the transition temperature TC or the response of our samples to AC magnetic susceptibility is relatively weak because of the weak magnetism.The relative intensities and energy of the observed features remain similar between all Cu-ABDC materials while their magnetism and electronic structures are very different.The reason is that the C K-edge XAS represents the transition from C 1s to unoccupied molecular orbitals with C 2p components (Figure S25a), and the magnetism of Cu-MOF are mainly related to the hybrid between Cu 3d orbitals.C K-edge XAS depends directly on the local coordination environment of C atoms, and the cleaving strategy mainly alters the local coordinated environment of the Cu atoms, resulting in slight changes in the overall lattice structure of the MOF.Therefore, the relative intensities and energy of the observed features in C K-edge XAS remain similar between all Cu-ABDC materials.Additionally, the C K-edge XAS spectra (Figure S25b) of Cu-ABDC MOFs can be roughly divided into two regions according to the incident photon energy due to the random orientation of the samples.][35][36] Specifically, the photon energy position of π* in our samples indicates electronic transitions from C 1s core level to delocalized π* energy level in Cu-ABDC MOFs, which suggests the delocalized π electrons are present in our MOFs.
To obtain the angle-dependent C K-edge XAS spectra, the Cu-MOF was carefully dissolved in ethanol and spin-coated on a silicon wafer to fabricate an orientedaggregated sample.Due to the π-π stacking interaction, the Cu-MOF sheets prefer to be deposited on silicon wafer with the benzene ring roughly perpendicular to the substrate.
In other words, the aromatic ring of Cu-MOF is along the normal of the sample.Hence, we can utilize the linear polarized X-ray to district the orbitals with π and σ symmetry (Figure S25c).The region before 293 eV where the intensity increases with the angle of incident light can be assigned to π* orbitals.The highest intensity was obtained when the angle between the electric field vector E of the incident light and the normal of the sample is 90 o , indicating that the final states are the orbitals perpendicular to the aromatic ring plane, e.g. the π orbitals.In higher energy σ* region above 293 eV, the intensity decreases with the angle of incident light.Here, the final state of transition is within the aromatic ring plane, which is with σ symmetry.][39] Supplementary Figure 26.Theoretical calculation results on a larger unit-cell for 1.4%-LS-Cu-ABDC (a) and 2.7%-LS-Cu-ABDC with the adjacent ligands (b), respectively.Red and blue iso-surfaces represent positive and negative spin densities, and the value is 0.0002 spins per bohr 3 , Copper, carbon, nitrogen, oxygen and hydrogen atoms are shown in blue, gray, yellow, red and white, respectively, and the axial solvent molecular are omitted for clarity in all structure models.
To confirm the long-range ferromagnetic order in our samples, theoretical calculation results on a larger unit-cell were performed.The spatial distribution of spin densities on a large unit-cell was shown in Figure S26.We can clearly see that the spin states are present in the benzene ring of the organic linkers, indicating that the delocalized π electrons in the organic linkers provide the bridge for the exchange interaction between Cu dimers.After the substitution of ligands, the spatial distribution of spin densities on the organic linkers is always present, and just gets weaker with the cleavage of the linkers.And the spin densities of the axial solvent molecules and partial atoms are omitted for clarity in all structure models.

10 Supplementary Figure 7 .Supplementary Figure 8 .
Cu K-edge EXAFS in R-space.Cu K-edge EXAFS curves including Cu foil and CuO as reference samples indicate the Cu-O coordination in Cu-ABDC.Wavelet transform (WT) analysis of Cu-MOFs.After filtering out the coordination of Cu-O in the first shell, the WT of Cu K-edge EXAFS data of Cu-MOFs show a maximum A at the cross-point of about RA = 2.1 Å/kA = 6.3 Å −1 , which is close to that of Cu foil, confirming that the second shell arises from the coordination of Cu-Cu, in agreement with the EXAFS fitting results.Furthermore, the maximum B at about RA = 3.4 Å/kA = 4.3 Å −1 may be from the multiple shell scattering or higher shell contribution of Cu-C coordination.

Supplementary Figure 11 .
Cu L-edge XAS.Cu L-edge XAS for Cu-ABDC including Cu2O and CuO as reference samples suggests +2 valence state of Cu ions in Cu-ABDC.

Supplementary Figure 13 .
Supplementary Figure 12. O K-edge XAS.O K-edge XAS of Cu-ABDC and the reference sample CuO.Two regions can be directly detected from O K-edge, a sharp π* region around 530 eV origins from the electrons excited into O 2p orbitals hybridized with Cu 3d, and the σ* region at about 540 eV is mainly from the electrons excited into O 2p orbitals that are hybridized with Cu 4sp and C 2sp. 15-17 O K-edge of Cu-ABDC is different from that of CuO because oxygen atoms connect not only with Cu ions but also with benzene ring, which further confirms the synthesis of Cu-ABDC and the absence of CuO.For O K-edge XAS in Fig. 2d, the linear pre-edge background was normalized to greatly exhibit the hybridization between O 2p and Cu 3d orbitals.Cu K-edge XANES curves.a, Cu K-edge XANES curves of Cu-MOFs and the reference samples.b, Cu K-edge first-derivative XANES curves of Cu-MOFs.The absorption edge positions of Cu-MOFs are close to that of CuO, indicating +2 valence state of Cu ions in Cu-MOFs.The decreased absorption intensities of white-line peaks at about 8996 eV and the lower-energy shifts of Cu Kedge first-derivative XANES further confirm that the electron occupation of Cu 3d states gradually increase with the increase of ligand cleavage.

Supplementary Figure 14 .Supplementary Figure 15 .Supplementary Figure 16 .
XPS survey analysis.XPS survey analysis reveals that there are only Cu, N, O and C elements in Cu-MOFs without other magnetic impurities.The plot of ln σ versus the reciprocal of the temperature (1/T) over the range from 320-373 K (a).The plot of ln (σ) versus T -1/4 over the temperature region 355-365 K (b).The dotted line is the experiment data, and the solid line is the fitting curve.I-V curves for Cu-ABDC collected at 320 K and 370 K.

Supplementary Figure 21 .
) vs. Ag/AgCl 2.7%-LS-Cu-ABDC 1.4%-LS-Cu-ABDC Cu-ABDC Supplementary Figure20.The M-H curves at different temperatures for Cu-MOFs.M-H curves of Cu-ABDC (a), 1.4%-LS-Cu-ABDC (b) and 2.7%-LS-Cu-ABDC (c), respectively.Distinct hysteresis loops were observed for 1.4%-and 2.7%-LS-Cu-ABDC MOFs in the temperature range from 5 to 300 K, as shown in FigureS20b-c.The remanent magnetization (Mr) is about 0.06 μB/Cu with the coercive field of about 170 Oe at 5 K, and 0.02 μB/Cu with a coercive field of 60 Oe at 300 K for 1.4%-Cu-ABDC MOF.For 2.7%-Cu-ABDC MOF, the Mr is about 0.01 μB/Cu and 0.003 μB/Cu at 5 K and 300 K, with a coercive field of about 100 Oe and 35 Oe, respectively.The relatively high remanent magnetization (about 12% at 5 K for 1.4%-Cu-ABDC MOF) at finite temperature indicates the presence of spontaneous magnetization, and hence the ferromagnetic order.AC magnetic susceptibility measurement of 1.4%-and 2.7%-LS-Cu-ABDC MOF.(a-b) Temperature variation of the real part of the ac susceptibility measurement (a) and imaginary part (b) at various frequencies from 1 Hz to 1000 Hz with Hac = 2 Oe and Hdc = 0 Oe from 50 to 360 K for 1.4%-LS-Cu-ABDC MOF.(c-d) Temperature variation of the real part of the ac susceptibility measurement (c) and imaginary part (d) at various frequencies from 1 Hz to 1000 Hz with Hac = 2 Oe and Hdc = 0 Oe from 50 to 360 K for 2.7%-LS-Cu-ABDC MOF.

Supplementary Figure 23 .
The calculated partial densities of states (PDOS) for 1.4%-LS-Cu-ABDC.The PDOS near the Fermi level of 1.4%-LS-Cu-ABDC display increased hybridization of the orbitals from Cu(3d) and C(2pz), indicating the presence of delocalized π electrons and π symmetry.

Supplementary Figure 25 .
Schematic illustration of an X-ray adsorption process of a diatomic molecule (a).C K-edge XAS of Cu-ABDC MOFs (b).The shadings were used to exhibit different regions.The schematic of π and σ bond (c).Angle-dependent XAS spectra of 1.4%-Cu-ABDC MOF at C K-edge (d).The measurement geometry is shown as an insert.